It is well known that certain patients experience loss of consciousness due to a sudden drop in blood pressure and/or heart rate. In many patients with such a cardioinhibitory component, intervention by pacing may help delay or reduce symptoms. The indicated forms of pacing to provide such help are DDD and DDD(R) pacing (hereinafter both referred to as DDD), where AV synchrony is maintained as such as possible; or DDI or DDI(R), or AV sequential pacing (hereinafter both referred to as DDI). In today's programmable pacemakers, there are provided algorithms for detecting sudden rate drop, but they generally are of limited use, and do not reliably detect sudden rate drop over the full range of physiological rates that a patient may experience. Such prior art solutions have generally been based on obtaining a sample of the patient's ongoing intrinsic rate history, and may incorporate some form of rate hysteresis to minimize the occasions when the pacemaker needs to override the intrinsic rate with delivered pace pulses.
The feature of rate hysteresis is well known in cardiac pacemakers. For example, many single chamber VVI pacemakers provide a form of rate hysteresis which is designed to prolong the next pacing escape interval which is set after a spontaneous, or natural ventricular beat. For example, the pacemaker may be programmed to pace at 70 beats per minute (bpm), but as long as natural beats are being sensed at rates above 70 bpm, the escape interval may be set to correspond to a somewhat lower rate, e.g., 60-65 bpm. The advantage of such hysteresis is that it enables the pacemaker to follow a mildly lower natural rhythm, i.e., one which might be just slightly below the lower rate limit (LRL) which has been programmed for pacing but still at a high enough rate that it is not necessary to override these natural beats with pacing. This achieves the advantages of maintaining the heart's AV synchrony as much as possible, and extending pacemaker longevity due to not delivering as many pace pulses. In dual chamber pacemakers, e.g. a DDD pacemaker where maximizing AV synchrony is an aim, providing hysteresis with respect to the atrial sensing enables increased tracking of natural atrial beats. Even in a DDI pacemaker, which does not track natural atrial beats as such, it remains desirable to maximize the number of atrial sense-ventricular sense (AS-VS) cycles, so as to maximize cardiac cycles where the heart is permitted to beat with its natural AV synchrony.
A known problem with hysteresis in cardiac pacing systems is that it can lead to excessive changes in rate. Usually a hysteresis rate is set which does not take into account the rate of the natural beats just prior to a sudden rate drop. With conventional hysteresis, whether the prior natural rate is close to the lower rate limit or far exceeds it, whenever the natural rate drops below the hysteresis rate, the pacemaker delivers a pace pulse at a rate corresponding to the hysteresis rate. Thus, a normal rate of 80-85 bpm could be followed by a pace pulse coming at an interval corresponding to a rate of only 60. Such a large drop in heart rate can have possible hemodynamic consequences, and the patient may experience a palpitation because of the sudden change. Further, in most hysteresis schemes, the pacing rate then goes to the pacing limit that had been set prior to losing the natural rate, and is maintained at such rate limit until a natural rate above this limit spontaneously reappears. This can mask the existence of an underlying natural rate just below the pacing limit, and aggravate the overall effect of loss of AV synchrony. It is thus seen that pacemaker performance would be improved by correlating the hysteresis function to the prior natural rate, and by improving the opportunity to recapture any underlying natural rate after pacing has taken over. There have been a number of different arrangements designed to enhance the hysteresis feature in cardiac pacemakers. See, for example, U.S. Pat. No. 5,016,630, which discloses a rate responsive pacemaker that varies the pacing rate, and provides a hysteresis interval which varies as a function of the escape interval corresponding to the dynamic pacing rate. It is also known to use the rate of change in the spontaneous intervals before hysteresis pacing to vary the escape interval after loss of the natural rate and takeover of hysteresis pacing. See U.S. Pat. Nos. 5,284,491 and 5,782,886. See also U.S. Pat. No. 3,921,642, which describes hysteresis "searching" for an underlying natural beat.
However, what remains needed in the art is a pacing system which optimizes hysteresis so as to detect a sudden rate drop anywhere across the rate spectrum, i.e., from the upper pacing limit to the lower pacing limit; and to respond to a detected sudden rate drop (SRD) with a pacing modality that gradually takes pacing rate to a safe rate while optimizing the opportunity to find an underlying natural rate and restore AV synchrony as quickly as possible. It is important to provide both reliable detection of SRD, and an optimized response to restore intrinsic physiological heart beats.